1. Field of the Invention
The present invention relates to a breaker employing a current-limiting device having PTC (Positive Temperature Coefficient) characteristics, and more particularly to a breaker for limiting and breaking a fault current using successive trips by electrically connecting a current-limiting device having PTC characteristics to a plurality of switches.
2. Description of the Related Art
Breakers are widely used for protecting lines and power equipments installed on the lines against a fault current such as a short circuit current in a power system such as a transmission system and a distribution system.
A conventional breaker includes a switch having a fixed contact point and a movable contact point and serially connected to a line for selective opening and closing, an extinction grid for extinguishing an arc generated in the switch while a fault current of the line is broken, and a movable contact point pivoting means for sensing a fault current and tripping the switch by making an angular motion of the movable contact point.
Seeing the operation of the conventional breaker, the fixed contact point and the movable contact point keep a contacted state between them at an ordinary time by using a certain force applied by the movable contact point pivoting means. However, if a fault current flows along the line, an electron repelling force generated between the fixed contact point and the movable contact point makes the movable contact point be rapidly released from the fixed contact point. Arc is generated between the released fixed and movable contact points, and the generated arc is operated toward the surrounding extinction grid, and then cooled and divided. The arc operated toward the extinction grid results in a voltage drop of the line, which limits a fault current flowing on the line, and the limited fault current is completely broken at an artificial current zero point by means of cooling and division of the arc.
Recently, various attempts have been made for realizing an efficient current-limiting and tripping operation of a breaker by connecting a mechanical switch with a current-limiting device having PTC characteristics that makes abrupt change of resistance according to temperature.
The current-limiting device is heated to increase its temperature abruptly by Joule heat when a fault current flows on a line, and its resistance value is abruptly increased when the temperature exceeds a threshold temperature. Accordingly, the fault current of the line is limited by the current-limiting device, and in this state the switch is mechanically operated to break the line.
If the line is broken, the temperature of the current-limiting device is dropped below the threshold temperature, and accordingly the resistance value of the current-limiting device is restored to its initial value. In addition, if a main cause of the fault current is removed and then the breaker is closed again, a common load current flows on the line.
The following prior art shows a breaker prepared by coupling a current-limiting device with a switch as mentioned above.
First, U.S. Pat. No. 2,639,357 discloses a technique of realizing a breaker by connecting a current-limiting device and switches in parallel. However, U.S. Pat. No. 2,639,357 has a drawback that a fault current is not suitably switched to the current-limiting device.
U.S. Pat. No. 4,878,038 discloses a technique of realizing a breaker by connecting a current-limiting device with switches in series. However, U.S. Pat. No. 4,878,038 has a problem that the current-limiting device connected with a line in series is continuously heated due to Joule heat at ordinary times, so a power loss is caused even when an ordinary load current flows.
U.S. Pat. No. 5,629,658 proposes a breaker operated using the successive trip mechanism by connecting a current-limiting device with a plurality of switches in parallel and in series in order to solve the problem of U.S. Pat. No. 4,878,038.
FIG. 1 shows a concept of the successive trip mechanism. As shown in FIG. 1, in the breaker of U.S. Pat. No. 5,629,658, a first switch 10 is connected to a current-limiting device 12 in parallel, and a second switch 14 is connected to the current-limiting device 12 in series. A load current at ordinary times flows through the first switch 10 having a relatively low resistance value. Thus, a problem of power loss caused by Joule heat generated in the current-limiting device 12 does not happen. Meanwhile, if a fault current such as a short circuit current occurs in a line L, the first switch 10 is firstly tripped due to the electron repelling force. According, the fault current flows through the second switch 14 and the current-limiting device 12. If the fault current flows on the current-limiting device 12, the fault current is limited due to the current limiting action of the current-limiting device 12. In addition, the second switch 14 is tripped due to the electron repelling force caused by the fault current and a second switch opening/closing tool separately prepared, so the fault current limited by the current-limiting device 12 is completely broken by the second switch 14.
Japanese Patent Publication No. H10-326554 proposes a more specific structure of a breaker adopting the successive trip mechanism.
FIG. 2 is a schematic view showing the breaker of H10-326554. As shown in FIG. 2, the breaker of H10-326554 includes a fixed arm 20 directly connected to a power source of a line and having a first fixed contact point 16 and a second fixed contact point 18 to which a PTC current-limiting device is fixed; and a movable arm 26 directly connected to a load of the line to rotate by an opening/closing tool and having a first movable contact point 22 contacting with the first fixed contact point 16, and a second movable contact point 24 contacting with the second fixed contact point 18.
The movable arm 26 is divided into a first movable arm 28 having elasticity and to which the first movable contact point 22 is attached, and a second movable arm 26 to which the second movable contact point 24 is attached. At ordinary times, the first contact points 16 and 22 and the second contact points 18 and 24 are electrically connected with each other, and a resistance between the first contact points 16 and 22 is smaller than a resistance between the second contact points 18 and 24, so most current flows through the first contact points 16 and 22 and the first movable arm 28.
If a fault such as a short circuit occurs in a line to flow a fault current through the line, an electron repelling force acts between the first fixed contact point 16 and the first movable contact point 22 so that the first movable arm 28 moves upward, which makes the first movable contact point 22 be released from the first fixed contact point 16. Accordingly, the fault current flows through the second fixed contact point 18 and the second movable contact point 24, and the fault current is limited by means of the current limiting action of the current-limiting device fixed to the second fixed contact point 24. At the same time, if the opening/closing tool detects the fault current and pivots the entire movable arm 26 upward, the fault current flowing between the second fixed contact point 18 and the second movable contact point 24 is completely broken.
However, the breaker of H10-326554 shows the following problems.
First, during the fault current breaking procedure of the breaker, an arc generated when the first contact points 16 and 22 are released may be operated toward the second fixed contact point 18, and also when the second contact points 18 and 24 are released, a serious arc is generated even between the second fixed contact point 16 and the second movable contact point 24. Arc causes a high temperature capable of melting metal or nonmetal material, so the second fixed contact point 24 composed of a PTC current-limiting device is apt to be melt, damaged or divided due to such an arc.
Second, when the breaker is closed, the second contact points 18 and 24 are firstly closed, and then the first contact points 16 and 22 are closed. Even in this breaker closing procedure, an arc is generated between the second contact points 18 and 24. Thus, the arc generated during the breaker closing procedure is apt to melt, damage or divide the second fixed contact point 24 composed of a PTC current-limiting device.
Third, the second fixed contact point 24 is composed of a PTC current-limiting device that is weaker than general contact point materials, so it is apt to be easily deformed or damaged. In addition, if the contact point itself is composed of a PTC current-limiting device, there is a drawback of shortening an electric life of the breaker as well as a mechanical life.
Fourth, a contact resistance between the first contact points 16 and 22 should be smaller than a contact resistance between the second contact points 18 and 24. However, if a contact resistance between the second contact points 18 and 24 is excessively great in comparison to a contact resistance between the first contact points 16 and 22, a fault current is not adequately switched to the second contact points 18 and 24 though the first contact points 16 and 22 are released before.
The breaker of H10-326554 configures the second fixed contact point 18 with a PTC current-limiting device. However, in this case, though a contact resistance between the second fixed contact point 18 and the second movable contact point 24 is increased to release the first contact points 16 and 22, a fault current may be not adequately switched toward the second contact points 18 and 24.
Fifth, a general contact point material is attached to the fixed arm 20 and the movable arm 26 by means of brazing. However, since the second fixed contact point 18 is composed of a PTC current-limiting device, it is impossible to use brazing for attachment of the contact points.
Sixth, the first movable arm 28 is made of metal with great elasticity. Thus, though the first movable contact point 22 and the first fixed contact point 16 attached to the first movable arm 28 are released due to an electron repelling force when a fault current occurs, the first movable arm 28 may be quickly closed again due to the elasticity of the first movable arm 28, which may resultantly limit the fault current insufficiently.